1. Field of the Invention
The present invention relates to filters and in particular to filters in a breathing circuit.
2. Summary of the Prior Art
Breathing circuits are well known in the art for various purposes, usually in medical applications, such as for providing breathing assistance and/or administering anaesthetics during surgical operations. Breathing circuits are also often used in providing assisted breathing during general medical care. These breathings circuits may for example extend from a ventilator or similar assisted breathing machine to a patient, and the circuit may include such auxiliary devices as in line humidifiers, carbon dioxide absorbers or antibacterial filters. These auxiliary components may be provided in the inspiratory or expiratory lines as appropriate.
U.S. Pat. No. 5,195,527, Hicks, shows an antibacterial filter for incorporation into a respiratory system, in particular in which a wall of filter media bisects a housing, being clamped between two parts forming the housing, one such part incorporating an inlet port and the other such part incorporating an outlet port such that gases flowing from the inlet port to the outlet port must pass through the antibacterial filter barrier. The filter disclosed in this publication includes flow deflectors at the entrances to the filter housing and flow directing vanes within the housing to disperse the gases flow and make effective use of the entire antibacterial filter surface. A filter of this type however has significant problems in handling a humidified gases stream, either humidified actively by incorporation of an in line humidifier in the respiratory system, or as a result of patient breathing. In particular condensation forms on the housing walls. This is exacerbated by condensation forming on the obstructions formed by the flow dispersing arrangements. Where the respiratory circuit is in use for significant periods of time condensation build up can be substantial and may pose a risk to patient health where there is any possibility of back flow to the patient. Furthermore a build up of condensation may lead to significant liquid wetting of the bacterial filter material and therefore a reduction in air permeability of the filter.
U.S. Pat. No. 4,727,871, Smargiassi et al, shows an exhalation system for connection in the exhalation path of a ventilator, in which a bacteria filter is mounted in an aluminium housing which is heated by a heating element with the purpose of reducing condensation in the bacterial filter and in other components of the system which are also mounted within the aluminium housing. This of course requires the provision of heating elements and means of controlling and supplying power to the heating elements which would together be a significant cost and which, particularly with the humid environment, have significant safety implications.
U.S. Pat. No. 5143060, Smith, shows an insulated carbon dioxide absorption system which incorporates a bacterial filter similar to that of Hicks within an insulated carbon dioxide absorption canister to utilise the heat generated in the reaction of the carbon dioxide with the granular absorbent material in warming and conditioning the respiratory gases for appropriate use with a patient. This device forms a substantial integrated component whose therefore associated costs do not lend itself to disposal, nor to use in non surgical or long term applications. With the bacterial filter disposed in an opaque housing there is the further disadvantage that detection of liquid build up, should any occur, is hindered.
It is therefore an object of the present invention to provide a bacteria filter which will at least go some way to overcoming the above disadvantages.
In a first aspect the invention may broadly be said to consist in a filter for a respiratory circuit comprising or including:
a housing having an inlet port and an outlet port for connection to respective breathing tubes and a gases space between said inlet port and said outlet port across which gases pass from said inlet port to said outlet port in use,
filter media spanning said gases space to divide said gases space into an inlet chamber and an outlet chamber such that gases passing across said gases space from said inlet port to said outlet port must pass through said filter media, and,
surrounding outer wall means surrounding said housing but for said inlet port and said outlet port and spaced from the wall of said housing to provide there between one or more air pockets between said outer wall means and said housing, which pockets together at least substantially surround said housing but for said inlet port and said outlet port to thereby insulate said housing (but for said inlet port and said outlet port) from ambient conditions.
In a further aspect the invention may broadly be said to consist in an insulating cover for a filter, said filter having an inlet port and an outlet port substantially coaxial therewith and spaced apart from said inlet port, and a filter media enclosing housing which first expands and then contracts as it extends between said inlet port and said outlet port, said insulating cover comprising or including a pair of cover members, each said cover member including a collar for fitting closely over one of said inlet port or said outlet port, a spacing member extending from said collar along the direction of their respective inlet or outlet port to space the collar away from the filter housing, and shell means extending to surround a respective portion of said housing, each said shell means having a periphery substantially at the broadest extent of said filter, the periphery of one said insulating cover being formed to interengage with the periphery of the other said insulating cover, such that the pair of covers may be fitted over their respective said inlet port or outlet port and brought together such that their peripheries engage, said spacing members keep the inlet port/outlet port surrounding collars spaced from said housing of said filter and said filter housing is enclosed within the insulating cover thus assembled.
In a still further aspect the invention may broadly be said to consist in a method of constructing a filter for a respiratory circuit comprising or including the steps:
1) clamping a web of filter media about its periphery between the periphery of a first housing forming part and the periphery of a second housing forming part,
2) fusing said first and second housing forming parts at their peripheries,
3) fitting a first cover forming shell over said first housing forming part such that an inlet port of said first housing part passes through an aperture in said first cover forming shell,
4) fitting a first cover forming shell over said first housing forming part such that an outlet port of said first housing part passes through an aperture in said first cover forming shell,
5) engaging said first and second cover forming shells such that said cover forming shells enclose the filter formed by the first and second housings and said web of filter media.